1. Field of the Invention
This invention relates generally to color television receivers, and more particularly is directed to improvements in color television receivers of the type employing a beam index color cathode ray tube.
2. Description of the Prior Art
In existing beam index color television receivers, a single electron beam scans the screen of a color cathode ray tube which, in its image or effective picture area, is provided with triads of red, green and blue color phosphor stripes extending vertically and being sequentially repeated in the horizontal scanning direction. The screen further has, on its inner surface, index phosphor stripes or elements which are spaced apart in the horizontal scanning direction in predetermined positional relation to the triads of color phosphor stripes, and which emit radiant energy when excited by the scanning beam. Such radiant energy emitted by the index elements causes a photo-detector or the like to produce a corresponding index signal which is employed to control the gating or switching of the color signals for modulating the single electron beam. Thus, by reference to the index signal, it is intended that the electron beam should be density-modulated or have its beam current determined by the red, green and blue primary color signals precisely when the single electron beam scans the color phosphor stripes which emit red, green and blue light, respectively.
In order to ensure that the switching of the color signals employed for modulating the single electron beam will be reliably and precisely synchronized with the scanning of the color phosphor stripes or elements by the electron beam, it is necessary that the index signal be always produced regardless of the content of the color video signal being applied to the receiver. However, in the case of existing beam index color television receivers, the electron beam current becomes so low as to provide inadequate excitation of the index phosphor stripes or elements when the applied video signal is at a black level, that is, during the reproduction of a black or dark portion of the television picture, with the result that only a weak or inconsistent index signal is provided and the switching of the color signals cannot be reliably achieved on the basis of such weak or inconsistent index signal. Furthermore, in existing beam index color television receivers, the characteristics, such as, the cut-off level, of the cathode ray tubes may vary from one tube to the next so as to cause undesirable variations and inconsistencies in the index signal obtained at the black level of the video signal. Such variations in the characteristics of the beam index color cathode ray tubes increase the likelihood of encountering an ineffective or unreliable index signal when the received video signal is at the black level.